Area: 8.87 hectares.
Description and Reasons for Notification:
Swanpool is a brackish lagoon located immediately west of Falmouth behind Swanpool beach. The site lies on alluvium underlain by Devonian sandstones and shales of the Falmouth and Mylor Series. The pool is fed by a small stream and seepages of the Swanvale catchment to the north. The pool was cut off from the sea by a sand and shingle bar deposited across its mouth at the end of the last Ice Age, forming a large freshwater lake. Fossil lake deposits from this era now underlie Swanvale to the north of the pool. The construction of an outlet culvert across the shingle bar to the beach in 1826 resulted in a considerable reduction in the extent of this freshwater lake and the tidal incursion of salt water. Swanpool is still connected to the sea via a tidal culvert which maintains brackish water conditions.
The semi-natural habitats fringing the pool, and in the lower catchment of the Swanvale represent successional stages from the former freshwater lake.
The open unvegetated water of the brackish lagoon is fringed by common reed Phragmites australis swamp and reed-beds, the occurrence of sea-club rush Scirpus maritimus is indicative of the brackish conditions. The reed-beds merge into wet willow woodland or Salix carr, dominated by grey willow Salix cinerea and occasional alder Alnus glutinosa. The ground flora comprises abundant yellow iris Iris pseudacorus, together with greater tussock-sedge Carex paniculata, water-mint Mentha aquatica, oppositive-leaved golden saxifrage Chrysoplenium oppositifolium and hemlock water-dropwort Oenanthe crocata. Much of Swanvale also supports wet willow carr vegetation, while in drier areas the canopy is dominated by ash Fraxinus excelsior, hawthorn Crataegus monogyna and pedunculate oak Quercus robur.
Swanpool* is of particular importance for two reasons, one as an example of a brackish lagoon, a rare habitat, covering only 770 ha. in Britain, and secondly as the only British location for the rare bryozoan, the trembling sea-mat Victorella pavida.
Swanpool is the largest saline lagoon in Cornwall and one of the eleven most important in Britain. The pool exhibits a well developed halocline which persists throughout the year and is maintained by the incursion of sea water on high water spring tides. The degree of salinity varies across the pool, creating a range of ecological conditions. Swanpool supports characteristic brackish lagoon species including Gammarus chevreuxi, Palaemonetes varians and Neomysis vulgaris, however its primary importance is for the occurrence of the rare trembling sea-mat,** found here at its only British location, growing mainly on submerged stones within the lagoon and the outlet culvert.
* Coastal lagoons are listed on:
Annex I of the EC Habitats and Species Directive (Council Directive 92/43/EEC).
** Trembling Sea-mat Victorella pavida listed on: Schedule 5 of the Wildlife and Countryside Act, 1981 Red Data Book No. 3: for Invertebrates other than Insects.